Known as a fastener element which is conventionally used for a slide fastener are a fastener element made of synthetic resin formed individually by injection-molding the synthetic resin to a fastener tape, a continuous fastener element formed by molding monofilaments in a coiled shape or a zigzag shape, a fastener element made of metal (hereinafter “metal element”) formed by clamping an approximately Y-shaped metal element material to the fastener tape and the like.
Further generally, as a metal element, a so-called single-sided metal element described in Japanese Patent Publication No. 2003-299509 (Patent Document 1), for example, or a so-called double-sided metal element described in Japanese Patent Publication No. 2009-34495 (Patent Document 2) are known.
A single-sided metal element 70 described in Patent Document 1 has, as shown in FIG. 19, a coupling head portion 70a formed by a pressing process and the like, a body portion 70b extending from the coupling head portion 70a, and a pair of leg portions 70c extending and branching as two-pronged from the body portion 70b. A coupling convex portion 70d is provided on one surface (top surface) of the coupling head portion 70a and a coupling concave portion not shown in Figures is provided on the other surface (bottom surface) of the coupling head portion 70a. 
Such a single-sided metal element 70 is manufactured by manufacturing Y-shaped element materials by slicing a long linear material (Y bar) whose cross section is Y-shaped and which is processed by multistage rolling process in a length direction at a desired thickness and by forming the coupling convex portion 70d and the coupling concave portion by pressing and deforming a part corresponding to the coupling head portion 70a of the obtained element material partially by the pressing process and the like.
The manufactured single-sided metal element 70 is attached to a fastener tape 71 by being pressed and plastically deformed in a direction in which both leg portions 70c come close to each other (inner side) in a state that a tape side edge part of the fastener tape 71 is inserted between a pair of leg portions 70c. 
When the single-sided metal element 70 attached to the fastener tape 71 is viewed from a tape front and rear direction, the single-sided metal element 70 has an appearance to have an rectangular-shaped base portion 72 and a protrusion portion 73 protruding from a tip end part of the base portion 72 (an end part on the coupling head portion 70a side) toward one direction of the tape length direction in a triangular shape.
In contrast, a double-sided metal element 80 described in Patent Document 2 has, as shown in FIGS. 20 and 21, a coupling head portion 80a, a body portion 80b extending from the coupling head portion 80a, and a pair of leg portions 80c extending and branching as two-pronged from the body portion 80b. Further, one coupling convex portion and one coupling concave portion are formed on both one surface (top surface) and the other surface (bottom surface) of the coupling head portion 80a, and the coupling head portion 80a has a symmetrical shape in an element thickness direction (a sliding direction of the slider).
The double-sided metal element 80 in Patent Document 2 is attached to a fastener tape 81 by being pressed and plastically deformed in a direction in which both leg portions 80c are close to each other (inner side) in a state that a tape side edge part of the fastener tape 81 is inserted between the pair of leg portions 80c. 
Here, for example, in a slide fastener having two sliders, when the slider slides in a top or a bottom direction with respect to an element row and for example in a case that the element row is formed of such single-sided metal elements 70 as in Patent Document 1, the operability is different between two sliders. On the other hand, in a case that the element row is formed of such double-sided metal elements 80 as in Patent Document 2, it can be prevented that the operability is different between two sliders.
Meanwhile, a fastener element made of synthetic resin is generally fixed to a fastener tape directly by injection molding. Therefore, by widening a fixing area of the fastener element with respect to the fastener tape, a fixing strength of the fastener element with respect to the fastener tape can be enhanced. Accordingly, in a conventional fastener element made of synthetic resin, an element width dimension of each fastener element is set to be large so as to secure the fixing strength of the fastener element stably.
In contrast, the above-mentioned metal element (single-sided and double-sided metal elements) is attached to the fastener tape by being pressed and plastically deformed in a direction in which both leg portions come close to each other (inner side) in a state that the tape side edge part of the fastener tape between a pair of leg portions is inserted between a pair of leg portions. Further, the metal element has more toughness than the fastener element made of synthetic resin. Therefore, the metal element can obtain enough fixing strength easily without setting the element width dimension large as in the case of the fastener element made of synthetic resin, for example.
Accordingly, the metal element can make an appearance of the element look slim by narrowing the element width dimension in comparison to the fastener element made of synthetic resin. Therefore, a product to which a slide fastener having the metal elements is attached (a fastener attached product) looks stylish or has a fashionable impression, which enables the appearance quality to be enhanced. On the other hand, since the metal element is heavier than the fastener element made of synthetic resin, it has a defect that weight of the fastener attached product increases.
In International Publication No. 2013/051149 (Patent Document 3), as a fastener element which has merits of both the conventional metal element and the fastener element made of synthetic resin, a fastener element made of synthetic resin which is lighter than the metal element and has such a slim appearance as the metal element (double-sided metal element) is described.
A fastener element 90 made of synthetic resin described in Patent Document 3 has, as shown in FIGS. 22 and 23, a central land portion 91 disposed at a center part in an element thickness direction, a first bulging portion 92 bulging from the central land portion 91 to a tape front surface side in the element thickness direction, and a second bulging portion 93 bulging from the central land portion 91 to a tape rear surface side in the element thickness direction.
The central land portion 91 has a body portion 91a fixed to a fastener tape 98, a neck portion 91b extending from the body portion 91a to a tape outward side in an element length direction, an oblong-shaped coupling head portion 91c further extending from the neck portion 91b in the element length direction, a shoulder portion 91d extending from the neck portion 91b in an element width direction, and a fin portion 91e extending from the body portion 91a to a tape inner side.
The first bulging portion 92 and the second bulging portion 93 are formed in a front-rear symmetrical shape about a reference surface positioned at a center of the fastener tape 98 in the tape thickness direction. In this case, the first bulging portion 92 has an upper end surface 94 facing to the element thickness direction and top and bottom sloped side surfaces 95 declining from a side edge of the upper end surface 94 to the central land portion 91.
The first bulging portion 92 has a quadrangular-shaped base body portion 96 disposed onto the body portion 91a of the central land portion 91 and an extending portion 97 extending from the base body portion 96 to the tape outward, and has a tapered element shape decreasing the element width dimension toward a tip end part of the extending portion 97 in viewing the whole first bulging portion 92 from an upper surface side.
Since the fastener element 90 having such a shape as Patent Document 3 is formed of synthetic resin, it is lighter than the metal fastener element. Further, the fastener element 90 secures the fixing strength to the fastener tape 98 at the body portion 91a of the central land portion 91.
At the same time, the fastener element 90 is formed so as to look slimmer than the conventional and general fastener element made of synthetic resin by having the first and second bulging portions 92, 93. Thus, it has such an appearance (visual) as the double-sided metal element.
Therefore, the slide fastener in which the fastener elements 90 in Patent Document 3 are attached to the fastener tape 98 looks stylish and has a fashionable impression as a slide fastener having the double-sided metal elements. At the same time, it is far lightweight in comparison to the slide fastener having the double-sided metal elements.